The year 315 BC marked a turning point for Roman military history when armies adopted the new formation during the Second Samnite War. This shift occurred after Roman forces suffered crushing defeats against Samnites in rugged terrain where phalanx formations proved useless.
Polybius outlined four distinct lines within the Roman fighting force consisting of velites, hastati, principes, and triarii. The front line contained young skirmishers while the rear held veteran soldiers with decades of service.
Roman formations maintained one maniple space between each unit to allow retreating soldiers to withdraw without disrupting those behind them. These open spaces lured rigid hoplite lines forward and disrupted their formation until they became disorganized surrounded prey for Roman swords.
Velites carried a sword javelins and a small shield called a parma while wearing no armor apart from a helmet often covered in animal skins. Triarii used thrusting spears instead of javelins like other infantrymen and wealthier soldiers could afford chain-mail cuirasses valued above 10,000 drachmas.
The manipular system faded from ancient sources following its long period of use before historians note that cohorts replaced maniples as organizational units within Roman armies. Polybius recorded legions consisting of 10 maniples of 120 hastati plus 10 maniples of 120 principes with half strength maniples of triarii containing 60 men each bringing total numbers to 4,500 men per legion.